Generally, thick film conductor paste compositions contain a conductive component, a binder, and an organic medium as the principal components. As the conductive component, fine powders of the noble metals palladium (Pd), platinum (Pt), gold (Au) and silver (Ag), or mixtures or alloys thereof, or oxides of palladium and silver, or mixtures thereof, have been widely used. Glass powder and various oxides are commonly used for the binder for these conductors, and the organic medium is an inert solution of polymers in an organic solvent or solvent mixture. The organic medium determines the application characteristics of the composition.
Fine powders of copper have also been used as the conductive component. Copper conductors can have very good electrical properties, good thermal conductivity and good solderability, and have a lower material cost compared to the noble metal conductors. Conductors based on copper need to be fired in an inert gas atmosphere to avoid oxidation, while conductors based on noble metals can simply be fired in air.
These thick film conductor pastes are typically applied to ceramic substrates by screen printing. Other methods of applying paste to a substrate such as spraying or brushing can also be used. The organic medium is adjusted to provide the proper viscosity and drying rate for the intended application. The conductive component and the inorganic binder, however, can be the same for all of these methods of application.
The screen printing process consists of forcing the thick film composition through a stencil screen onto the substrate with a squeegee. The open pattern in the stencil screen defines the pattern that will be printed onto the substrate. Solvents are removed by drying, and the dried prints are then fired to remove the residual resin and to densify the conductor. Ceramic substrates such as aluminum oxide or aluminum nitride are commonly used.
Improved copper conductor compositions are needed to provide a copper conductor with low resistivity and the necessary adhesion to the substrate.